Duplex filter systems are known. In a duplex filter system the secondary filter takes flow when the primary filter is dirty or while the primary filter is being serviced. Such secondary backup is more efficient and less costly than allowing an expensive machine to be inoperative while its only filter for its hydraulic or lubrication system is being serviced.
Conventional duplex filter systems, however, have a number of disadvantages. Known systems require the attention of an operator to manually shift a multi-port plug valve to direct fluid from the primary filter to the secondary filter when an indicator shows the primary filter is sufficiently dirty or plugged. In addition, some duplex filter systems include bypass valves in one or both the primary and secondary filters which allow passage of dirty fluid whenever the particular filter is neglected and becomes sufficiently dirty or plugged. Such a system allows the dirty fluid to pass into the machine, a situation which may then cause minor, if not major, damage.
Other duplex filter systems have no bypass valves, but have low to medium collapse filter elements. If the filter is neglected, at a differential pressure across the filter in the range of 100 to 300 PSI or so, the element will collapse and again dirty fluid passes into the machine or use device. Still other duplex filter systems have no bypass valves, but solve the filter element collapse problem by using expensive filter elements which collapse at a high differential pressure, for example, 2000 to 3000 PSI. In addition to the high cost of such elements, a neglected filter in this case causes power drawn by the pump to increase excessively until the filter element collapses.
Thus, to the thoughtful engineer, present disadvantages of known duplex filter systems often outweigh the primary advantage of having a backup secondary filter for reducing or eliminating machine down time.
The invention approaches duplexing by eliminating the precision multi-port plug valving traditionally provided as a lever operated means of switching from the primary filter to the secondary filter. These multi-port valves normally are required to be unique to each manufacturer. In its place the invention advantageously uses a low cost commercially available ball valve and a novel regulating valve to accomplish the same shifting purpose and to gain in addition the automatic shift from primary filter to secondary filter.